Intermolecular Energy Transfer Revealed with Atomic Resolution

Abstract

Intermolecular energy transfer is an essential dynamic process in photosynthesis and various energy-converting devices. So far, optical spectroscopy has been widely used to investigate the dynamic phenomenon. However, the spatial resolution of the conventional method is limited, and nanoscale spatial features in the energy transfer are still unknown. Here we demonstrate an atomic-scale investigation of energy transfers between a free-base phthalocyanine and a magnesium phthalocyanine (H₂Pc and MgPc) using newly developed absorption/emission spectroscopy based on scanning tunneling microscopy (STM). A luminescence signal from H₂Pc was detected while locally exciting a nearby MgPc with the tunnelling current of STM, clearly indicating an energy transfer from MgPc to H₂Pc. The mechanism of the energy transfer is proven to be resonance energy transfer because charge transfer is prohibited by the energy-level alignment at the MgPc-H₂Pc heterojunction. The fundamental insights into energy transfers pave the path for realizing excitonic circuits with molecular architectures on solid surfaces.